The goal of this research project is to understand the nature of mouse embryonic and adult stem cells and to identify genes that are responsible for the maintenance of cellular pluripotency. We have been conducting global gene expression profiling with the mouse embryonic cDNA microarrays developed in our laboratory. In our previous work, we performed large-scale gene expression profiling on embryo-derived stem cell lines to identify molecular signatures of pluripotency and lineage specificity. Analysis of pluripotent embryonic stem (ES) cells, extraembryonic-restricted trophoblast stem (TS) cells, and terminally-differentiated mouse embryo fibroblast (MEF) cells identified expression profiles unique to each cell type, as well as genes common only to ES and TS cells. The identification of genes that are specifically expressed in ES cells has provided an important first step for understanding the pluripotency of stem cells. These genes can be used as markers for pluripotent stem cells. We have identified embryonal stem cell-specific gene 1 (Esg1) as such and have shown tight coexpression of Esg1 and Oct3/4, which is the most well-known transcription factor governing the cellular pluripotency. We are currently studying the function of this gene in ES cells and preimplantation mouse embryos. To extend this work and extract the common features of stem cells, we have been examining the gene expression profiles of mouse ES and other stem cells cultured in a variety of condition or after the manipulation of specific genes in ES cells. Especially, the gene expression profiling of ES cells, in which the Oct3/4 level is controlled by tetracycline-inducible system, has identified a number of downstream target genes of Oct3/4 and begins to provide the gene regulatory pathways involved in Oct3/4 genes.